Changing Date Graduation of a graph

Is this what you mean?

library(tidyverse)

# Sample data on a copy/paste friendly format (replace with your actual data frame)
monthly <- data.frame(
  MonthlyMax = c(70.4,74.7,77.5,82.9,88.5,89.5,89.5,
                 91.9,87.6,85.1,81.9,75.6,72,75.5,76.2,82.9,87.8,89.9,
                 91.7,89.1,90.4,85.6,81.3,75.5,73.5,71.3,75.4,82.7,
                 85.8,88.8),
  MonthlyMin = c(55.2,56.3,61.8,63.1,77.8,83.9,81.5,
                 81,79.7,69.6,64.7,-99,40.2,48.9,-99,71,-99,80.2,83,
                 80.6,-99,75.7,-99,58.4,58.4,51.7,60.9,66.9,75.9,80.2),
        Date = as.Date(c("2015-01-01","2015-02-01","2015-03-01",
                 "2015-04-01","2015-05-01","2015-06-01","2015-07-01",
                 "2015-08-01","2015-09-01","2015-10-01","2015-11-01","2015-12-01",
                 "2016-01-01","2016-02-01","2016-03-01","2016-04-01",
                 "2016-05-01","2016-06-01","2016-07-01","2016-08-01","2016-09-01",
                 "2016-10-01","2016-11-01","2016-12-01","2017-01-01",
                 "2017-02-01","2017-03-01","2017-04-01","2017-05-01","2017-06-01"))
)

monthly %>%
    gather(Legend, Value, -Date) %>% 
    ggplot(aes(x = Date, y = Value, fill = Legend)) +
    geom_bar(position = "dodge", stat = "identity") +
    scale_x_date(date_breaks = "month", date_labels = "%Y-%b") +
    theme_bw() +
    theme(axis.text.x = element_text(angle = 45, hjust = 1, vjust = 1))

Created on 2020-11-22 by the reprex package (v0.3.0.9001)

Note: Next time please provide a proper REPRoducible EXample (reprex) illustrating your issue.